The present invention relates to a non-noble metal combustion catalyst for reduction of carbon monoxide, hydrocarbons, and nitrous oxides from the exhaust gases of internal combustion engines and industrial vapor emissions.
Internal combustion engines are a primary power source for numerous applications. However, combustion by-products carried by their exhaust gas, primarily hydrocarbons, carbon monoxide and nitrous oxides, have undesired effects upon the environment. Similarly, noxious by-products from various manufacturing processes such as paint production can have damaging environmental effects. One common method to remove these pollutants is a catalytic converter through which the engine exhaust or industrial vapor flows before being expelled into the atmosphere. These converters, although of many specific constructions, generally consist of a mixture of one or more noble metals, such as palladium, rhodium, or platinum, disposed in one or more layers on porous substrate. Platinum and palladium are used primarily for oxidation reactions to remove hydrocarbons and carbon monoxide, while rhodium acts as a main catalytic active element for nitrous oxide reduction.
In specific, in a automobile exhaust converter, the nitrogen oxide (NO.sub.x) may be reduced according to the following reactions: EQU 2NO.sub.x +2XCO.fwdarw.N.sub.2 +2XCO.sub.2 ( 1) EQU 2NO.sub.x +2XH.sub.2 .fwdarw.N.sub.2 +2XH.sub.2 O (2)
It is well known that the reaction (2) is a more efficient means for reduction of NO.sub.x. If the reduction of NO.sub.x follows the reaction (2), the conversion efficiency of NO.sub.x may be dramatically increased. However, the main reaction follows reaction (1) route when conventional noble metal three way converters are used (reference to W. C. Hecker and R. B. Breneman, "The Effect of Weight Loading and Reduction Temperature on Rh/Silicon Catalysts for NO Reduction by CO"; F. C. M. J. M. Van Delft et al., "An AES Investigation of the Reactivity of Pt, Rh, and Various Pt-Rh Alloy Surfaces Towards O.sub.2, NO, Co, and H.sub.2 "; P. W. Goodman et al., "Mechanisms of Carbon Monoxide Oxidation and Nitric Oxide Reduction Reaction Over Single Crystal and Supported Rhodium Catalysts: High Pressure Rate Explained Using Ultrahigh Vacuum Surface Science").
An inherent problem with noble metal catalytic converters is that an increasing scarcity of noble metals increases their cost and, moreover, the decreasing supply cannot indefinitely keep up with demand. In order to solve the foregoing difficulties, various non-noble metal catalysts have been proposed as replacements for the noble metal catalysts. However, none of these non-noble metal catalysts possess adequate catalytic action necessary to meet modern anti-smog requirements and they are unable to stand the heavy load of space velocity encountered. Also, since modern engines possess very high exhaust temperatures, the low thermal stability of the prior art non-noble metal catalytic converters significantly reduces their working life. The low thermal stability of the prior art non-noble converter is due in part to their susceptibility to temperature induced sintering and recrystallization, which effects the reduction of the ratio of catalyzing surface area to catalyst weight. Furthermore, at high temperatures some non-noble metal oxides can react with one another to form inactive compounds which serve no useful purpose.
In short, none of the non-noble metal catalysts of prior art possess an adequate combination of good three-way catalytic activity, the capability to endure the load of high space velocity, low ignition temperature, high thermal resistance, and strong mechanical strength. Moreover, none exhibit the necessary working life to be economical replacements for current noble metal catalytic converters.
Examples of the prior art non-noble metal catalysts are described in U.S. Pat. Nos. 3,956,189 and 3,914,389. These patents teach oxides of metals such as copper, cobalt, and manganese deposited on various conventional carries, or teach specified compounds such as lanthanum copper manganese oxide, without a carrier.
Also, U.S. Pat. Nos. 4,637,995; 3,498,927; 4,631,268, and U.K. Patent Publication No. 2,012,616 describe monolithic support structures of various types as potential carriers for various metal-containing catalysts. However, these catalyst all suffer from shortcomings which render them inadequate and impractical. Namely, the shortcomings include the thermal instability discussed hereinabove, high minimum catalyzing or igniting temperature, and low space velocity requirement. Furthermore, these prior art non-noble metal catalysts do not convert nitrous oxide so as to achieve three way catalys operation.
Some of these prior art monolithic catalyst supports provide a high surface area carrier phase as an integral discontinuous phase with the ceramic matrix itself. Lachman (U.S. Pat. No. 4,631,267) and De Angelis (U.S. Pat. No. 4,637,995) teach such a process and material. The Lachman references state that if solid starting materials are used, it is necessary to well mix the materials before forming the final product. Many mixing procedures are time-consuming and require specialized equipment.